Microencapsulation is the envelopment of small, solid particles, liquid droplets, or gas bubbles with a coating, usually a continuous coating. Many terms are used to describe the contents of a microcapsule such as active agent, active core, core material, fill, internal phase, nucleus, and payload. The coating material used to form the outer surface of the microcapsule is called a coating, membrane, shell, or wall. It may be an organic polymer, hydrocolloid, sugar, wax, metal, or inorganic oxide. Microcapsules usually fall in the size range of between 1 and 2000 microns, although smaller and larger sizes are known.
Complex coacervation and interfacial polymerization are two well known processes for the formation of microcapsules. Complex coacervation is carried out in aqueous solution and is used to encapsulate water-immiscible liquids or water-insoluble solids. In the complex coacervation of gelatin and gum arabic, for example, the water-immiscible substance to be encapsulated is dispersed in a warm, gelatin solution. Gum arabic and water are added, the pH of the aqueous phase adjusted to about 4, and a liquid complex coacervate of gelatin and gum arabic is formed. As long as the coacervate adsorbs on the substance being encapsulated, a coating of complex coacervate surrounds the dispersed droplets or particles of water-insoluble substances to form microcapsules. The gelatin is then crosslinked, typically with an aldehyde such as formaldehyde or glutaraldehyde. A variety of other crosslinking agents are known, including polyfunctional carbodiimides, anyhdrides, and aziridines.
Aldehydes provide only slight crosslinking, however, so the shell walls of the microcapsule remain hydrophilic, swelling in water. Consequently, gelatin microcapsules may be difficult to dry without aggregation. Furthermore, some of the aldehydes, such as formaldehyde, are environmentally undesirable, partially because of their odor.
In interfacial polymerization reactions, the fill is typically a liquid rather than a solid. Interfacial polymerization involves the reaction of various monomers at the interface between two immiscible liquid phases to form a film of polymer that encapsulates the disperse phase. The monomers diffuse together and rapidly polymerize at the interface of the two phases to form a thin coating. The degree of polymerization can be controlled by the reactivity of the monomers chosen, their concentration, the composition of either phase vehicle, and by the temperature of the system.
Microcapsules produced through interfacial polymerization having shell walls composed of polyamides, polyureas, polyurethanes, and polyesters are known; see U.S. Pat. Nos. 3,516,941, 3,860,565, 4,056,610, and 4,756,906. In some instances the shell walls of these conventional microcapsules are very porous and consequently disperse their fill too rapidly for some applications. Therefore, the microcapsules may have to be post-crosslinked with such crosslinking agents as polyfunctional aziridines. The crosslinking provides shell walls with greater structural integrity and reduced porosity. Of course, an obvious disadvantage to post-crosslinking or curing is that it adds another step to the microcapsule production process.
One of the difficulties in producing microcapsules through interfacial polymerization reactions is that the shell wall precursors used can react too slowly with each other at the interfacial boundary of the two different phases containing the reactants. When the latter occurs, the reactants sometimes diffuse past one another and no polymerization reaction will take place. Alternatively, if polymerization does take place, it proceeds too slowly and microcapsules with shell walls having poor structural properties and solubility characteristics will result. As one skilled in the art realizes, it can be very difficult to achieve successful interfacial polymerization between two reactants even though they are known to readily react with one another at room or higher temperatures in solution or other types of polymerization reactions which do not occur at an interfacial boundary. Therefore, in general, the fact that two materials will react readily in the latter environment is not necessarily an indication that they will react well in interfacial polymerization reactions.
It was against this background that Applicants began their search for microcapsules which could be produced by an efficient process and yet which also possessed shell walls having excellent structural properties and solubility characteristics. In the course of their investigations, Applicants discovered that microcapsules containing shell walls which are formed by using polyfunctional aziridines as one of the primary reactants in the microcapsule production process during interfacial polymerization possess such characteristics.
Although polyfunctional aziridines have been used as a crosslinking agent for the shell walls of gelatin compositions as disclosed in U.S. Pat. No. 2,950,197, there has been no disclosure to date of which Applicants are aware that polyfunctional aziridines have been used as one of the primary shell forming components in microcapsules. When the polyfunctional aziridines have been used as crosslinking agents (i.e. typically at levels of less than about 5 wt %), they have not been incorporated into the polymeric backbone of the shell wall of the microcapsule as opposed to when they are employed as a primary shell-forming component. Although polyfunctional aziridines are known to react with a wide variety of other polyfunctional organic compounds at either room or higher temperatures, there has been no indication from the literature as to whether or not polyfunctional aziridines would be suitable as reactants in interfacial polymerization where the reaction must take place very quickly in order to get desirable shell wall properties as explained before. Typical crosslinking reactions involving aziridines and other crosslinkers generally proceed over several hours or days, not seconds or minutes as required in an interfacial polymerization reaction.